Surface acoustic wave (SAW) devices have been utilized in a number of sensing applications, and have been adapted for use with telecommunications equipment for the realization of delay lines and filters. Filters and delay lines require very good temperature and stress stability. Proper design, material choice (e.g., quartz cuts) and packaging techniques have lead to the realization of devices having improved performances. Note that such “improved performances” are principally related to the stability of surface wave velocities, which must remain as stable as possible within a specified range of temperature and stress.
The modification of the surface wave velocity due to such aforementioned factors can be utilized for measuring purposes. SAW-based devices have been utilized to measure strain, temperature as well as chemical changes that take place in materials during exposure to chemical agents.
One application of SAW-based sensors, for example, involves the remote interrogation of a SAW-based device by RF wireless signals, due to the fact that a SAW sensor can store energy provided by an RF signal. In this manner, SAW-based sensors do not require an energy source to function, which can be of great interest in some applications.
SAW-based devices are ideal for pressure sensing applications. One important feature of a SAW-based pressure sensor involves the implementation of a proper sensor diaphragm that can withstand the strain or stress induced by pressure. Prior Art SAW-based pressure configurations utilize different materials, such as glue, solder or epoxy adhesive in order to bond a quartz plate, which can be referred to as a “base,” having a given thickness to an appropriate substrate.
The quartz plate (i.e., the “base”) can sustain the SAW device. The sensitivity of the sensor is related to the thickness of the diaphragm—i.e., equal to the thickness of the quartz plate—as well as to its effective exposed area to pressure. The substrate can be metal, ceramic or other suitable materials. The area of the diaphragm subjected to pressure can be obtained by boring or drilling the substrate. The packaging of such devices should be accomplished with much care in order to ensure an accurate alignment of the sensor to the diaphragm.
Another problem observed in the aforementioned configurations arises from the mismatch between the substrate and the quartz temperature coefficient of expansion (TCE) that induces supplementary and uncontrollable stress into diaphragm. One conventional solution involves the use of a quartz plate instead of metal plate as substrate material.
The two quartz plates—the first one that sustains the sensor (“base” plate) and the second that forms the substrate and is drilled in order to delimit the diaphragm area—can be glued or bonded together. In such a configuration, the stress induced by the mismatch between the TCE can be reduced but, again, the drilling of the quartz substrate and the alignment of the two plates are not accurate. Moreover, drilling a quartz plate is time consuming and can induce defects into the plate. Such defects decrease the strength of the material.
In an improved design, the substrate plate possesses an upper cavity that delimits the diaphragm area and a trough-via that allows pressure to enter into the cavity. The two plates can be bonded together utilizing glass frit technology. Such a method surpasses the previous problems related to stress induces by temperature mismatch and to drilling. The alignment of the diaphragm, however, remains a problem.
Another design involves a carefully configured mechanical support for the quartz plate (i.e., the “base” plate) to avoid the stress induced by the mismatch in TCE between the quartz plate and the substrate. In such a situation, the diaphragm is mechanically deformed by the metallic cover of the sensor. Careful mechanical set-up increases the cost of this sensor.
A second important factor related to the realization of a SAW-based pressure sensor is the necessity of protecting the surface of propagation of the mechanical wave from the atmosphere. The majority of the aforementioned solutions ensure this requirement by providing a vacuum reference chamber, which can be configured during packaging.
Other methods have been implemented to protect the surface of the SAW sensor (not necessarily a pressure sensor). Metallic, ceramic, glass or quartz covers has been utilized to seal the sensor. The bonding of covers with a quartz plate supporting the sensor (i.e., the “base”) can be realized by a metal solder, epoxy resin or a glass frit. All such techniques involve some mechanical problems. A polymer sealing method has also been proposed, but is generally not suitable for pressure sensor applications.
It is believed that a solution to the aforementioned problems associated with conventional SAW-based sensor technologies, and particular SAW-based pressure sensors, involves the use of so-called micro electromechanical system (MEMS) technology. Micro Electro Mechanical System (MEMS) is a technology that implements mechanical and electrical parts, using semiconductor-processing techniques. A conventional MEMS device generally includes floating driving parts that are movable over a substrate in order for the device fabricated using MEMS technology to perform mechanical operations. In general, MEMS technology involves the integration of mechanical components, sensors, actuators and other electronic and/or mechanical elements on a common substrate (e.g., silicon) through the use of micro-fabrication or micro-machining manufacturing techniques